The invention relates to a centrifugal fluid pump assembly for pumping a medical fluid, typically blood.
In modem medical treatment, centrifugal blood pumps are often used in artificial heart/lung units for extracorporeal blood circulation. Centrifugal pumps of the magnetic coupling type wherein a driving torque from an external motor is transmitted to the impeller through magnetic coupling are commonly used because the physical communication between the blood chamber of the pump and the exterior can be completely excluded to prevent invasion of bacteria.
In general, centrifugal blood pumps include a housing having a blood inlet port and a blood outlet port and an impeller accommodated for rotation in the housing for feeding blood by a centrifugal force developed during rotation. The impeller having magnetic pieces of permanent magnet disposed therein is rotated by a rotational torque generating mechanism which includes a rotor having magnets for attracting the magnetic pieces of the impeller and a motor for rotating the rotor.
FIG. 11 shows an example of an artificial heart/lung unit using the blood pump. An artificial heart/lung unit 100 comprises a blood pump 101, an artificial lung 102, and a pressure gauge 103 serving as a measuring means. When the blood pump 101 is of centrifugal type, it is necessary to provide the artificial heart/lung unit 100 with a flow meter 104. The measuring devices such as the pressure gauge 103 and the flow meter 104 are expensive. Further, it is necessary to provide the artificial heart/lung unit 100 with increased number of connection portions of the measuring devices, which increases the possibility of coagulation of blood.
When the blood pump is used for the artificial heart/lung unit, blood is gathered at intervals of certain period of time to examine the state of the blood by a device. In the examination, a hematocrit value and the viscosity of the blood are changed by the use of hemodilution agent. The viscosity of the blood also changes in dependence on the temperature of the blood. A change in the viscosity of the blood changes the flow velocity distribution of the blood flowing in a flow passage. The change in hematocrit value and the flow velocity distribution are factors of errors of various flow meters.
That is, as the flow meter for the artificial heart/lung unit, an electromagnetic flow meter and an ultrasonic Doppler flow meter are used. In the electromagnetic flow meter, the dielectric constant of fluid affects an output voltage greatly, and the hematocrit value changes the dielectric constant of fluid greatly. Thus, a change of the hematocrit value is the factor a cause of errors. In the ultrasonic Doppler flow meter, the change in a flow rate distribution is a main factor cause of errors. Because the viscosity of blood changes flow rate distribution, the viscosity of the blood affects measurement accuracy. It is possible to correct the values by verifying a flow meter for each hematocrit value and viscosity. Because the hematocrit value and the blood viscosity are measured by the batch which requires blood-gathering, it has been impossible to perform such a correction in real time.
When a blood pump is embedded in the human body, it is difficult to install a flow meter and a pressure gauge therein because the space for accommodating them is small.
It is an object of the invention to a centrifugal fluid pump assembly having a viscosity calculation function capable of calculating the viscosity uf fluid easily and reliably.